Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone baking mat for easy removal.

In a small saucepan over medium heat, combine the blueberries, lemon juice, and sweetener (if using). Cook, stirring occasionally, until the blueberries break down and form a thick, jam-like consistency (about 5–7 minutes). Remove from heat and let cool slightly while you prepare the cloud bread batter.

In a blender or food processor, blend the cottage cheese, egg yolks, vanilla extract, and sweetener until smooth and creamy. Set aside.

In a clean, dry mixing bowl, beat the egg whites and cream of tartar with a hand mixer or stand mixer on high speed until stiff peaks form (about 3–5 minutes). The egg whites should hold their shape when you lift the beaters.

Gently fold one-third of the whipped egg whites into the cottage cheese mixture to lighten it. Carefully fold in the remaining egg whites in two additions, being careful not to deflate the mixture. Use a spatula and fold gently until just combined.

Spoon mounds of the cloud bread batter onto the prepared baking sheet, spacing them about 2 inches apart. Use the back of a spoon to create a slight well in the center of each mound. Spoon a small amount of the blueberry mixture into the wells and use a toothpick to swirl it gently into the batter for a marbled effect.

Bake in the preheated oven for 20–25 minutes, or until the cloud breads are golden brown and set. Let them cool on the baking sheet for 5 minutes before transferring to a wire rack.

Serve warm or at room temperature. Pair with butter, cream cheese, or additional fresh blueberries for extra indulgence.